1. Field of the Invention
The present invention relates to improved methods of forming spaced fractures in a subterranean zone penetrated by a horizontal well bore.
2. Description of the Prior Art
Subterranean hydrocarbon containing formations penetrated by well bores are commonly stimulated by creating fractures therein. That is, a fracturing fluid such as a gelled aqueous fluid is pumped into the formation at a rate and pressure such that fractures are created and extended therein. A propping material such as sand is typically deposited in the fractures so that they are prevented from completely closing when the formation is returned to production and provide flow passages through which hydrocarbons readily flow to the well bore.
Subterranean formations are also commonly treated with aqueous acid solutions to stimulate the production of hydrocarbons therefrom. One such treatment known as "fracture-acidizing" involves the formation of one or more fractures in the formation and the introduction of an aqueous acid solution into the fractures to etch the fracture faces whereby flow channels are formed when the fractures close. The aqueous acid solution also enlarges the pore spaces in the fracture faces which increases production of hydrocarbons into the flow channels.
In carrying out fracturing and fracture-acidizing treatments in subterranean formations penetrated by open hole horizontal well bores, sealing mechanisms such as one or more packers have been utilized to isolate spaced portions of the formations along the horizontal well bores so that a plurality of spaced fractures can be formed therein. However, such sealing mechanisms are often incapable of containing the fracturing fluid utilized at the required fracturing pressure. Even when the sealing mechanisms are capable of isolating a formation to be fractured, the use and installation of the sealing mechanisms are time consuming and add considerable expense to the fracturing process.
It has been the more recent practice in wells containing open hole horizontal well bores, particularly in sandstone and carbonate formations, to dispense with the use of sealing mechanisms and to pump aqueous fracturing fluids with or without acids into the horizontal open hole sections at high rates. In this process, the fracturing fluid creates fractures in an uncontrolled manner with the largest volume of the fracturing fluid generally entering one or more fractures close to the point of entry of the fracturing fluid into the horizontal well bore. Consequently, only a few fractures are often formed in the well bore with very few or none being formed along the length of the well bore.
Thus, while the recently utilized fracturing procedure is of low cost, can be performed rapidly and allows the rapid flow back of fluids after fracturing, the procedure often does not result in as great an increase in hydrocarbon production as would be the case if the fractures were spaced along the length of the horizontal well bore in the producing formation. Therefore, there is a need for improved methods of fracturing subterranean formations penetrated by open hole horizontal well bores. Further, there is a need for such improved methods whereby a plurality of spaced fractures can be produced along the length of an open hole horizontal well bore in a subterranean formation without using sealing mechanisms. Further, there is a need for such methods which are relatively economical, can be rapidly performed and allow the rapid flow back of fluids after the fracturing procedure.